To that end, MIT in 2009 established a research group called the Concrete Sustainability Hub, with support from the cement industry. This month, the Concrete Sustainability Hub issued two major reports — one on concrete pavements, the second on concrete buildings — that examine in detail those products’ life-cycle costs, in both money and greenhouse gas emissions. The group’s principal researchers say these are the most comprehensive and transparent (all their data and assumptions are open) analyses ever attempted.
CONCRETE PAVEMENTS
“We wanted to put out a methodology that’s more or less comprehensive, looking at everything in the whole life cycle,” says Nicholas Santero, a research scientist in MIT’s Department of Civil and Environmental Engineering and the lead author of the 100-page report on pavements.
To study the effect of pavement stiffness, the team used computer models, as previous efforts to directly measure physical effects had been “all over the map” because the differences are so small, Santero says. Essentially, less-stiff roads that give more as a vehicle passes over create a small indentation where the wheels are, so that to a very small extent the car is always going “uphill” and uses more fuel. Stiffer roads, such as those made from concrete rather than asphalt, can thus provide a slight boost in efficiency.
Because there are wide variations in usage patterns and climate conditions experienced by different roads, the MIT team looked at 12 specific types of roads — ranging from interstate highways to local surface roads — to carry out their analysis.
Along with devising a method that others can apply to evaluating choices for a particular construction project, the team came up with some specific suggestions of actions that could improve a road’s life-cycle costs, emissions or both:
- Increase maintenance work on roadways to keep the surface smoother, thus improving the gas mileage of the cars and trucks that use it. For example, instead of scheduling road maintenance every 20 years, do it every 10 years.
- When pavement is replaced, pulverizing the old concrete and leaving it exposed for at least a year causes it to absorb carbon dioxide from the air, helping to cancel out part of the emissions released when the cement was produced.
- Even the color of a road can mitigate its overall effect on Earth’s climate: Lighter roads reflect more sunlight, while darker ones absorb it and get hotter. Just as white roofs can help to reduce warming of the climate, so can lighter pavements — which can be produced by adding lighter-colored aggregate (gravel or crushed rock) to the concrete mixture.
- Reassess the design criteria for road construction, to account for local and regional differences. Most specifications are now generic, which results in over-engineering many roads, making them stronger than they need to be. Simply reducing the paving thickness in places where this can be done without degrading performance could significantly reduce the amount of cement used, thus reducing both costs and emissions.
- Add more fly ash, a waste product scrubbed from the emissions of coal-fired powerplants, to the concrete mix. This material is already widely used, but increasing its use could displace more cement powder, which is a highly energy-intensive material to produce.
Adding up these measures, Santero says, it’s possible to reduce the overall carbon emissions associated with concrete pavements by about 50 percent, relatively easily.
CONCRETE BUILDINGS
The construction and operation of buildings accounts for approximately 40 percent of all U.S. emissions of greenhouse gases. The most-used building material in the world, concrete, is used to construct many of the nation’s homes and office buildings — but the new MIT report says a variety of measures could drastically reduce, and ultimately even eliminate, the carbon footprint of most new concrete buildings, as well as some older ones.
The new report outlines some of those measures, analyzing buildings’ energy use and carbon emissions over their entire life cycle of construction, use and eventual demolition. The researchers say the report provides the most detailed and open accountings ever undertaken of the full life cycle of buildings.
The report reflects nearly two years of work by the MIT team, says lead author John Ochsendorf, associate professor of civil and environmental engineering and architecture. His group’s life-cycle analysis extends “all the way down to details of where the components come from, and how were they transported.” In doing so, researchers were able to “quantify emissions and potential savings, and also put a cost on them,” he says.
Not only are there significant savings possible in the energy use of buildings and their associated emissions, but some of these are cost-free: “There are steps to reduce carbon emissions that save money, that pay back owners,” Ochsendorf says.
Typically today, concrete is used in construction purely for its structural properties, but by harnessing concrete’s thermal properties for passive solar storage, the material could greatly reduce a building’s energy needs. For example, by designing windows and overhangs so concrete is exposed to sunlight during the winter, the material can effectively store heat during the day and release it at night. In addition, pipes embedded in concrete floors, walls and ceilings can be used for both heating and cooling, providing greater efficiency as well as greater comfort than systems that rely on heating the air in the room, the report says.
“Ochsendorf says. Yet governments around the world are already starting to set up requirements for significant reductions in buildings’ carbon footprints, and the Intergovernmental Panel on Climate Change has identified buildings as the most cost-effective sector for implementing policy to reduce greenhouse gas emissions. But, Ochsendorf says, in order to show that real reductions are taking place, first you need a reliable assessment of the emissions associated with existing buildings, and a methodology for comparing those with newer ones. The new MIT report provides that, he says.
While many people have attempted to carry out life-cycle analyses, the process is far more complex than it may appear, explains Concrete Sustainability Hub Director Hamlin Jennings. “A great deal of life-cycle analysis is based on opinion,” he says — not necessarily because the data are uncertain, but because some decisions on what to factor in are inherently subjective.
Another example is the fuel used for kilns that make cement — whose use, in many cases, actually provides an environmental benefit. “It turns out that often a significant amount of that fuel might otherwise be waste material, and some of it even toxic waste,” Jennings says. “The kiln operates at a very high temperature and decomposes it. So do we just not consider it? All these fuzzy areas have to be looked at carefully.”
The MIT report, he says, attempts to provide a more complete approach than others have: “All the boundaries were looked at carefully, and we included the cradle-to-grave analysis in one package.” The team “produced a model that is transparent and can be tested by others,” he adds.
Already, the American Institute of Architects has embraced an initiative, called the 2030 Challenge, to spur dramatic reductions in buildings’ energy use and emissions; many cities and organizations have already agreed to its goals, which call for a 60 percent reduction in emissions (compared to the existing average) right away, and a 100 percent reduction by 2030. In other words, by then buildings should have no net energy consumption at all — which, amazingly, is already feasible today, Ochsendorf says.
The Concrete Sustainability Hub is funded by the Portland Cement Association and the Ready Mixed Concrete Research & Education Foundation.
